Compressed gas is becoming an increasingly popular choice for transportation of fuels. As more and more vehicles use compressed gas as fuel, more emphasis needs to be put on the emissions of the fuel systems of such vehicles. Currently, there is a requirement from compressed gas fuel systems that addresses the safety risks of gas permeation from such a system. There is, however, currently no standard that addresses the environmental concerns of compressed gas permeation. When such a standard is introduced, it is likely to be much more restrictive than the current emissions standard.
The vast majority of the compressed gas tanks used on transportation vehicles today is designed to store compressed methane gas (CH4) and is of type III or type IV. There is however, for environmental reasons, a desire to move to compressed hydrogen (H2) as fuel. There are currently no production vehicles being fueled by compressed hydrogen.
Because of the larger molecule size of methane, compared to hydrogen, conventional type II, III and IV tanks meet the current permeation requirements when filled with compressed methane gas. For compressed hydrogen, these conventional tanks do however not necessarily meet the requirements, especially if, as expected, more restrictive requirements are introduced.
Storage tanks have been classified in different categories. Type II tanks concern all steel tanks, which are generally rather heavy and therefore not favored in relation with vehicles. Type III tanks are composite tanks with aluminum liners. These tanks show excellent emission results but are rather expensive. Type IV tanks are composite tanks with polymer liners. Type IV tanks also have very good emission results but, compared to the type III tanks, these tanks have the further advantage of being lighter and less expensive. Therefore, type IV tanks are the most likely candidates for compressed hydrogen storage tanks.
One example of a type IV storage tank is shown in WO 07/110399 in the name of the Applicant, which teaches a method for manufacturing a pressure vessel having an outer reinforcing layer (outer shell) and an inner liner comprising hemispherical end caps butt welded on a cylindrical body. The conventional blow molding technique, which is well known in the field of regular fuel tanks, can be used for making the cylindrical body. However, a pinch is created at least at one extremity of the liner and is then removed to create a cylindrical body with an open end. The removal step of the pinch and the welding of the end cap imply an increase of the manufacturing time and wasting of material. Consequently such manufacturing method has the disadvantage of not promoting high volume production.
Additionally, the pinch that may result from the compression of extremities of the parison has to resist to high pressure values, in particular in applications for pressure vessels. Under the effect of internal pressure on the pinch, this latter may separate and a leak may result in the pressure vessel.